1. Field of the Invention
Example embodiment(s) of the present invention are related in general to a computer-implemented method and system for designing a nuclear reactor core which satisfies NRC steady-state and transient licensing requirements.
2. Description of the Related Art
A core of a nuclear reactor such as boiling water reactor (BWR) or pressurized water reactor (PWR) has several hundred individual fuel bundles of fuel rods (BWR) or groups of fuel rods (PWR) that have different characteristics. These bundles (or fuel rod groups) are arranged so that interaction between rods within a fuel bundle, and between fuel bundles satisfies all regulatory and reactor design constraints, including governmental (licensing) and customer-specified constraints. Additionally for a BWR, the control mechanisms, e.g. rod pattern (control blade) design and core flow, must be determined so as to optimize core cycle energy. Core cycle energy is the amount of energy that a reactor core generates before the core needs to be refreshed with new fuel elements, such as is done at an outage.
Traditionally, reactor core and operational strategy design for a BWR proceeds in several phases. The first phase, fuel cycle development, typically occurs approximately 9 months in advance of the fuel outage and involves certain assumptions about key operating parameters, such as operating limit critical power ratio (OLMCPR), the determination of which depends on the ultimate design result.
The second phase reactor core and operational strategy design, known as reference loading pattern (RLP) development, provides a refinement of the fuel cycle design in preparation for licensing. The RLP forms the basis of all licensing (transient) calculations to be performed to assure the license-ability as well as safety of the plant for the upcoming fuel cycle. The RLP involves some modification of previous operating parameter assumptions, especially if there has been a marked change in the energy plan from previous cycles, introduction of new fuel products, or a change in operational strategy. These modifications in assumptions result in changes to the final core design and operational strategy. As a result, significant differences from the original fuel cycle can result, with a potential for negative impact on economics, margin, or flexibility of operations.
The reasons for the two phase approach to design and licensing stem from the requirement to use numerous computer codes that have been developed for evaluation of accident and transient scenarios that determine the design basis for the plant. Many of the actual licensing limits, especially those related to thermal performance of the fuel, are condensed into a series of design curves that are readily evaluated by a 3D core simulator. Other licensing limits can be ignored in the design phase since these licensing limits are not considered design limiting. These ignored licensing limits are only evaluated during the post-design licensing phase, not during reactor core design.
The existing design process requires a designer to be cognizant of potential licensing problems a priori in the design and licensing process. Past experiences with a particular plant and fuel cycle design approach could suggest additional ‘soft constraints’ introduced by the designer to hedge against future problems in the licensing phase. Even so, violation of licensing criteria during the licensing phase nevertheless occurs, in which case, additional design iterations need to be performed. These additional iterations involve ‘detuning’ a design, which can involve adding additional fresh bundles, adding additional cost down at end of cycle, and/or reducing flexibility of operations. Often, assumptions made by core designers with regard to design limits impact the aggressiveness with regard to maximizing fuel cycle efficiency.
Additionally, no assistance is provided via the iterative design process in order to guide a designer toward a more favorable design solution. The conventional core design process typically looks at a subset of the licensing limits (i.e., those considered to be most limiting in the core design and translatable into a pseudo-steady state design limit readily evaluated by the 3D simulation code). The responsible engineer's experience and intuition are the sole means of performing design changes, with the goal of improving the overall core design solution. However, other transient licensing constraints which also depend on the core design (and which are ignored during the design process), can and do in fact become limiting.